Add mandatory name attribute to OML model

src/xopp2oml/template.oml

@@ -13,7 +13,9 @@ description <{{output_namespace}}#> as {{shorthand}} {
   uses <{{namespaces.dict}}#> as dict
   {%- endif %}
 
-  ci model : xopp:Model []
+  ci model : xopp:Model [
+    object_diagram:hasName "{{shorthand}}"
+  ]
 
   ci file : xopp:File [
     xopp:hasCreator {{ file.creator | to_oml_string_literal }}